PROCESS FOR INCREASING OPACITY OF PRECIPITATED CALCIUM CARBONATE

Abstract

The present invention relates to a process for preparing an aqueous suspension comprising precipitated calcium carbonate. The invention further relates to an aqueous suspension comprising precipitated calcium carbonate as well as a partially dewatered or essentially completely dried precipitated calcium carbonate obtainable by the process and the use of the aqueous suspension comprising precipitated calcium carbonate and/or the partially dewatered or essentially completely dried precipitated calcium carbonate in paper making, paper coating, plastic, agricultural, adhesives, sealants and/or paint applications.

Claims

1. A process for preparing an aqueous suspension comprising precipitated calcium carbonate, the process comprising the steps of: a) providing at least one calcium source being essentially insoluble in water; b) providing at least one water-soluble manganese source; c) providing at least one carbonate source, and d) carbonating the at least one calcium source of step a) with the at least one carbonate source of step c) in the presence of the at least one water-soluble manganese source of step b) to convert the calcium source at least partially into precipitated calcium carbonate.

2. The process according to claim 1, wherein the at least one calcium source of step a) is selected from the group comprising calcium oxide, calcined dolomite, dolomite, limestone, calcium carbonate, hydrated calcium oxide, calcium arsenate, calcium arsenite, calcium benzoate, calcium chromate, calcium citrate, calcium fluoride, calcium phosphate, calcium silicate, calcium sulfate, calcium sulphide, calcium tartrate and mixtures thereof, and most preferably the at least one calcium source of step a) is hydrated calcium oxide

3. The process according to claim 1, wherein the at least one calcium source of step a) is provided in an aqueous environment, preferably in an aqueous environment comprising the at least one calcium source of step a) in an amount of from 1.0 to 60.0 wt.-%, preferably from 5.0 to 50.0 wt.-%, more preferably from 8.0 to 30.0 wt.-% and most preferably from 10.0 to 20.0 wt.-%, based on the total weight of the aqueous environment and the at least one calcium source of step a).

4. The process according to claim 1, wherein the at least one water-soluble manganese source of step b) is selected from a manganese(II) source, a manganese(III) source and mixtures thereof, preferably from a manganese(II) source.

5. The process according to claim 4, wherein the at least one water-soluble manganese source of step b) is at least one manganese(II) source selected from the group comprising manganese(II) sulfate, manganese(II) hydroxide, manganese(II) chloride, manganese(II) acetate, manganese(II) arsenite, manganese(II) benzoate, manganese(II) bromide, manganese(II) carbonate, manganese(II) chlorate, manganese(II) citrate, manganese(II) formate, manganese(II) iodide, manganese(II) nitrate, manganese(II) oxalate, manganese(II) phosphate, manganese(II) tartrate, manganese(II) thiocyanate and mixtures thereof.

6. The process according to claim 1, wherein the at least one carbonate source of step c) is derived from a) a gaseous CO.sub.2 source in the presence of water, preferably a CO.sub.2 comprising gas in the presence of water, and/or b) a carbonate-comprising anion, preferably a carbonate-comprising anion being selected from the group consisting of sodium carbonate, potassium carbonate, sodium hydrogen carbonate, potassium hydrogen carbonate or mixtures thereof.

7. The process according to claim 1, wherein the at least one water-soluble manganese source of step b) is present in carbonating step d) in an amount from 10 ppm to 1 000 ppm (d/d), preferably from 20 ppm to 900 ppm (d/d), more preferably from 50 ppm to 800 ppm (d/d), even more preferably from 100 ppm to 700 ppm (d/d) and most preferably from 200 ppm to 600 ppm, based on the total dry weight of the at least one calcium source of step a).

8. The process according to claim 1, wherein the at least one calcium source of step a) is added to an aqueous environment to convert the calcium source at least partially into calcium cations before carbonating step d) is carried out.

9. The process according to claim 1, wherein the at least one water-soluble manganese source of step b) is added before and/or during and/or after adding the at least one calcium source of step a) to the aqueous medium, preferably the at least one water-soluble manganese source of step b) is added after adding the at least one calcium source of step a) to the aqueous medium.

10. The process according to claim 1, wherein the starting temperature of step d) is adjusted to a temperature of between 5? C. and 80? C. and most preferably to a temperature of between 10? C. and 70? C.

11. The process according to claim 1, wherein the aqueous suspension comprising precipitated calcium carbonate obtained has solids content from 5.0 to 40.0 wt.-%, preferably from 10.0 to 30.0 wt.-%, and most preferably from 15.0 to 20.0 wt.-%, based on the total weight of the aqueous suspension.

12. The process according to claim 1, wherein the precipitated calcium carbonate obtained by the process a) comprises particles having a weight median particle size d.sub.50 value of less than 15.0 ?m, more preferably of less than 10.0 ?m, even more preferably of less than 5.0 ?m and most preferably of less than 3.0 ?m, and/or b) has a specific BET surface area of 1.0 m.sup.2/g to 50.0 m.sup.2/g, more preferably of 3.0 m.sup.2/g to 35.0 m.sup.2/g and most preferably of 4.0 m.sup.2/g to 20.0 m.sup.2/g, and/or c) has a degree of whiteness R457 of at least 85%, more preferably of at least 90%, even more preferably of between 85 and 95% and most preferably of between 90 and 95%, and/or d) has a yellowness index of at least 1.5, more preferably of at least 2.0, even more preferably of between 2.0 and 4.5 and most preferably of between 2.5 and 4.0.

13. The process according to claim 1, wherein the process further comprises step e) of drying or dewatering the obtained aqueous suspension comprising precipitated calcium carbonate to remove at least a portion of water to obtain a partially dewatered precipitated calcium carbonate or to remove more than 99 wt.-% of water to obtain an essentially completely dried precipitated calcium carbonate.

14. The process according to claim 13, wherein the partially dewatered precipitated calcium carbonate is in form of an aqueous suspension having solids content from 20.0 to 70.0 wt.-%, preferably from 25.0 to 65.0 wt.-%, and most preferably from 30.0 to 60.0 wt.-%, based on the total weight of the aqueous suspension.

15. The process according to claim 13, wherein the process further comprises step f) of treating the essentially completely dried precipitated calcium carbonate of step e) with at least one hydrophobizing agent, preferably an aliphatic carboxylic acid having a total amount of carbon atoms from C4 to C24 and/or at least one mono-substituted succinic anhydride consisting of succinic anhydride mono-substituted with a group selected from a linear, branched, aliphatic and cyclic group having a total amount of carbon atoms from C2 to C30 in the substituent and/or a phosphoric acid ester blend of one or more phosphoric acid mono-ester and one or more phosphoric di-ester, to obtain a precipitated calcium carbonate comprising on at least a part of the accessible surface area a treatment layer comprising the hydrophobizing agent.

16. Aqueous suspension comprising precipitated calcium carbonate obtainable by the process according to claim 1.

17. Partially dewatered or essentially completely dried precipitated calcium carbonate obtainable by the process according to claim 14.

18. A paper, paper coating, plastic, agricultural product, adhesive, sealant or paint comprising the precipitated calcium carbonate according to claim 16 and/or a partially dewatered or essentially completely dried precipitated calcium carbonate thereof.

19. The paper, paper coating, plastic, agricultural product, adhesive, sealant or paint of claim 18, which is paper selected from lightweight papers such as cigarette papers, thin print papers, SC magazine papers and the like, and printing or writing papers such as copy papers, uncoated- and coated offset papers and the like.

Description

EXAMPLES

[0186] Measurement Methods

[0187] The following measurement methods are used to evaluate the parameters given in the examples and claims.

[0188] Brookfield Viscosity

[0189] The Brookfield-viscosity of a slurry was determined with a Brookfield Viscometer type RVT equipped with a LV-3 spindle at a speed of 100 rpm and room temperature (20?3? C.).

[0190] BET Specific Surface Area of a Material

[0191] Throughout the present document, the specific surface area (in m.sup.2/g) of a particulate material was determined using the BET method (using nitrogen as adsorbing gas), which is well known to the skilled man (ISO 9277:1995). The total surface area (in m.sup.2) of the particulate material is then obtained by multiplication of the specific surface area and the mass (in g) of the particulate material. The method and the instrument are known to the skilled person and are commonly used to determine the specific surface of particulate materials.

[0192] Particle Size Distribution (Mass % Particles with a Size <X) and Weight Median Particle Size (d.sub.50) of a Particulate Material

[0193] Throughout the present invention, d.sub.50 is the weight median particle diameter, i.e. representing the particle size at which 50.0 wt.-% of the particles are coarser or finer.

[0194] The weight median particle diameter was measured using the sedimentation method. Particle mass was measured directly via X-ray absorption. The sedimentation method measures the gravity-induced settling rates of different size particles in a liquid with known properties. The measurement is made with a Sedigraph? III Plus of Micromeritics Instrument Corporation. The method and the instrument are known to the skilled person and are commonly used to determine grain size of fillers and pigments. The measurement was carried out in an aqueous solution of 0.1 wt.-% of a cationic dispersant (Polystabil CGU supplied by Stockhausen GmbH, Germany). The samples were dispersed using a high speed stirrer and supersonic.

[0195] Alternatively, the weight median particle size and the average particle size mass distribution of a particulate material was determined via laser diffraction, i.e. the light from a laser passes through a suspension and the particle size distribution is calculated from the resulting diffraction pattern. The measurement was made with a HELOS Particle size analyzer and the Software WINDOX of Sympatec GmbH. The method is well known to the skilled person and is commonly used to determine the particle size distribution of particulate materials. The measurement was carried out with a homogenous aqueous slurry having a solids content of about 20.0 wt.-%, based on the total weight of the aqueous slurry. The samples were shaken well until the aqueous slurry is homogenously mixed, i.e. until no sediments were left in the slurry before measuring. As reference material SiC-F1200 of Sympatec GmbH was used, which was prepared as a homogeneous aqueous slurry having a solids content of 20.0 wt.-%, based on the total weight of the aqueous slurry. The samples were homogeneously mixed by using an ultrasonic bath.

[0196] Pigment Brightness and Paper Opacity

[0197] Pigment brightness and paper opacity were measured using an ELREPHO 3000 from the company Datacolor according to ISO 2469:1994 (DIN 53145-2:2000 and DIN 53146:2000).

[0198] Whiteness R457 and Yellowness Index Measurement

[0199] Whiteness and yellowness index were determined according to norm TAPPI T452/ISO 247. Glossiness was determined according to DIN 54 502/TAPPI 75.

[0200] PH of an Aqueous Suspension

[0201] The pH of the aqueous suspension is measured using a standard pH-meter at approximately 22? C.

[0202] Solids Content of an Aqueous Suspension

[0203] The suspension solids content (also known as dry weight) is determined using a Moisture Analyser HR73 commercialized by Mettler-Toledo with the following settings: temperature of 120? C., automatic switch off 3, standard drying, 5-20 g of suspension.

[0204] d/d

[0205] The term d/d refers to the dry amount based on the dry amount of the solid material.

[0206] Filler Content (Ash Content)

[0207] The filler content in the samples such as handsheets and cigarette papers was determined by burning a defined amount of the sample in a muffle furnace heated to 570? C. After the burning was completed, the residue was transferred in a desiccator and allowed to cool down. When room temperature was reached, the weight of the residue was measured and the mass was related to the initially measured weight of the sample.

[0208] Handsheet Study

[0209] For the handsheet study, eucalyptus kraft pulp (obtained from SAPPI Schweiz AG, Biberist Switzerland) refined to 30? SR was used. An aqueous suspension was prepared by diluting 60 g (dry) pulp and an amount of the corresponding precipitated calcium carbonate required to achieve the desired filler content to a total volume of 10 L. The obtained suspension was stirred for at least 12 min. Subsequently, 450 ml of the obtained suspension were mixed with 0.06 wt.-% (based on dry weight) of polyacrylamide (Polymin 1530, commercially available from BASF, Ludwigshafen, Germany) as retention aid. Then, handsheets of the 80 g/m.sup.2 were formed using a Rapid-Kothen hand sheet former. On each sheet a cover sheet was placed. The sheets were then dried using the Rapid-Kothen drier by applying vacuum such that a constant pressure over the whole sheet is obtained and a temperature of 105? C. for 6 min.

[0210] The filler content of handsheets was determined after constant weight was reached upon rapid incineration of a quarter handsheet sample at 570? C. in a muffle furnace. After burning was completed, the residue was transferred in a desiccator and allowed to cool down. When room temperature was reached, the weight of the residue was measured and the mass was correlated with the initial weight of the quarter hand sheet. To determine the grammage, handsheets were kept at 23? C. and 50% relative humidity for 24 hours.

[0211] VIS/UV Spectroscopy

[0212] The VIS spectroscopy results were obtained by using a Perkin Elmer Lambda 2 UV/VIS spectrometer and usage of Integrating Sphere as accessory for reflective-measurement. That is to say, the UV rays are directly reflected on the filler particle surface, the light is then entering the integrating sphere (I-sphere), where it multiply reflected and ultimately ends up in the detector. The samples were measured by using a sample holder under usage of 8? angle against aluminum as zero line. Furthermore, it is to be noted that the measurement is only started after a stable measurement signal is reached (up to the third decimal place). The scanning rate was 15 nm/min. The samples in form of 40 mm pellets were immediately measured after pressing at 10 t (by using a press of PerkinElmer Inc., Germany) and 40 t (by using a pelletizing press of HERZOG Maschinenfabrik GmbH & Co. KG, Germany).

Example 1Comparative Example

[0213] The following comparative example illustrates the preparation of precipitated calcium carbonate by a process of the prior art. Said process is carried out by slaking calcium oxide and contacting the obtained hydrated calcium oxide with gaseous CO.sub.2 to convert the hydrated calcium oxide into precipitated calcium carbonate.

[0214] 200 kg of PCC grade high-purity quicklime CaO (Kalkwerk LEUBE, Golling/Austria) was added to 1 700 liters of 40? C. tap water in a stirred slaking reactor. Before slaking, 0.66 kg of a 30 wt.-% aqueous solution of sodium citrate-was added to the slaking water. The quicklime was slaked for 30 min under continuous stirring and the resulting slurry of hydrated calcium oxide (milk of lime) was adjusted to 13.5 wt.-% solids content, based on the total weight of the slurry, via dilution with water having a temperature of 60? C. The resulting slurry was then screened on a 200 ?m screen to remove grit.

[0215] The subsequent carbonation was conducted in a 2 000 liters baffled cylindrical stainless steel reactor equipped with a gassing agitator, a stainless steel carbonation tube to direct a carbon dioxide/air gas stream to the impeller and probes for monitoring the pH and conductivity of the suspension. 1 800 liters of the 13.5 wt.-%, based on the total weight of the slurry, aqueous suspension of hydrated calcium oxide obtained in the slaking step as described above were added to the reactor and adjusted to a temperature of 50? C. Prior to carbonation, 1.7 kg of sucrose was added to the milk of lime and the reaction mixture was homogenized for 5 min via agitation.

[0216] A gas mixture containing 20% by volume of CO.sub.2 and 80% by volume of air was then bubbled upwards through the slurry at a volumetric flow of 200 Nm.sup.3/h under vigorous agitation. During the carbonation, the temperature of the reaction mixture was not controlled and allowed to rise due to the heat generated in the exothermic precipitation reaction. The carbonation was continued until the electrical conductivity suddenly dropped and passed through a minimum indicating that the carbonation was substantially complete. The introduction of the CO.sub.2-containing gas was continued for another 10 min before the introduction of gas was stopped. Carbonation time, calculated from start of gas introduction to time of conductivity minimum, was 103 min. The slurry was then screened on a 45 ?m screen and the screened product was recovered as an aqueous slurry of CaCO.sub.3.

[0217] Characteristics and physical properties are given in column A of table 1.

Example 2Inventive Example

[0218] The following illustrative example of the invention involves the preparation of precipitated calcium carbonate by carbonating hydrated calcium oxide with gaseous CO.sub.2 to convert the hydrated calcium oxide into precipitated calcium carbonate, wherein the carbonation is carried out in the presence of a water-soluble manganese(II) source.

[0219] Example 2 was carried out in a similar manner as Example 1, with the exception that a parent solution of manganese(II) sulfate monohydrate was added to the diluted slurry of hydrated calcium oxide having solids content of 13.5 wt.-%, based on the total weight of the slurry, in an amount of 300 ppm (d/d), based on the total dry weight of the hydrated calcium oxide, after slaking but before the carbonating was started.

[0220] The parent solution of manganese(II) sulfate monohydrate having a concentration of manganese(II) sulfate monohydrate of about 30%, based on the total weight of the parent solution, was prepared by dissolving 300 g of manganese(II) sulfate monohydrate per liter water. The product was recovered as an aqueous suspension.

[0221] Characteristics and physical properties are given in column B of table 1.

Example 3Inventive Example

[0222] Example 3 was carried out in a similar manner as Example 2, with the exception that the parent solution of manganese(II) sulfate monohydrate was added to the hydrated calcium oxide suspension in an amount of 500 ppm (d/d), based on the total dry weight of the hydrated calcium oxide.

[0223] The product was recovered as an aqueous suspension. Characteristics and physical properties are given in column C of table 1.

TABLE-US-00001 TABLE 1 column A B C example 1 (reference) 2 3 specific surface area BET m.sup.2/g 10.8 11.1 11.2 PSD weight median particle size d.sub.50 HELOS Particle size analyzer ?m 2.91 2.96 2.91 Sedigraph III Plus ?m 1.68 1.61 1.59 Brightness (DIN 53145) L* 99.08 98.49 98.23 a* ?0.02 0.18 0.25 b* 0.86 1.32 1.52 R457 (ISO 2469) % 96.50 94.36 93.42 Yellowness index (DIN 6167) % 1.57 2.59 3.01 Solids content Slurry % 16.4 16.4 16.4 Viscosity Slurry (Brookfield mPas 26 25 27 100 rpm) pH Slurry 7.6 7.8 7.7

[0224] As can be gathered from the data shown in table 1, the inventive method especially leads to precipitated calcium carbonate having a reduced brightness and whiteness R457 and an increased yellowness index and thus results in a precipitated calcium carbonate with increased or optimized opacity.

[0225] Furthermore, UV- and visible light absorption determined by spectroscopy analysis is significantly higher for Example 3, which has been prepared by adding 500 ppm (Example 3) of manganese(II) sulfate monohydrate to the hydrated calcium oxide suspension, compared to the reference sample (Example 1) prepared in the absence of a water-soluble manganese source. In general, it can be gathered that the intensity of the spectral signal has in the middle of the VIS spectrum its lowest magnitude and rises stronger in the range of lower VIS spectrum (in direction of UV range) than at the beginning of the IR-range (800 nm). In the whole VIS spectrum, Example 3 (500 ppm of manganese(II) sulfate monohydrate) has the highest absorption, except in the range from 400 to 450 nm, where the reference sample (Example 1) has the highest absorption.

[0226] The VIS/UV spectroscopy results are shown in FIG. 1.

Example 4Paper Study

[0227] Handsheet studies were done simulating copy paper comprising a precipitated calcium carbonate prepared in accordance with the inventive process and a calcium carbonate prepared in accordance with the prior art, respectively. In particular, the precipitated calcium carbonate prepared in accordance with the inventive process was prepared as described above for Example 2, i.e. a parent solution of manganese(II) sulfate monohydrate was added to a diluted slurry of hydrated calcium oxide having solids content of 13.5 wt.-%, based on the total weight of the slurry, in an amount of 300 ppm (d/d) after slaking but before the carbonating was started. Each paper had a weight of 80 g/m.sup.2. The CaCO.sub.3 content of the prepared papers was between 10 and 25 wt.-%, based on the total weight of the paper.

[0228] Characteristics and physical properties of the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are given in table 2.

TABLE-US-00002 TABLE 2 example 1 2 (prior art) (inventive) specific surface area BET m.sup.2/g 10.3 10.6 PSD weight median particle size d.sub.50 HELOS Particle size analyzer ?m 2.41 2.17 Sedigraph III Plus ?m 1.39 1.37 R457 (ISO 2469) % 97.1 94.6 Yellowness index (DIN 6167) % 1.4 2.4 Solids content slurry % 34.9 36.9 Viscosity Slurry (Brookfield mPas 295 496 100 rpm) pH slurry 8.3 8.3

[0229] Characteristics such as the filler content, opacity as well as whiteness R457 of the obtained papers comprising the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are given in table 3.

TABLE-US-00003 TABLE 3 filler content Opacity Whiteness R457 Paper comprising [wt.-%] [%] [%] Prior art PCC 11.86 92.85 88.71 16.24 93.48 89.21 20.57 93.88 90.15 Inventive PCC 10.89 92.63 88.25 15.42 93.69 89.08 19.58 94.06 89.71

[0230] As can be also gathered from the data shown in FIG. 2, the inventive method especially leads to precipitated calcium carbonate achieving an increased or optimized opacity when implemented as filler material in paper compared to a prior art calcium carbonate.

Example 5Cigarette Paper Study

[0231] This example discloses the preparation of cigarette paper on an industrial cigarette paper machine. The design and setting of such a machine is well known to the skilled person.

[0232] For comparison reasons, cigarette paper comprising a precipitated calcium carbonate prepared in accordance with the inventive process and cigarette paper comprising a calcium carbonate prepared in accordance with the prior art were produced. Each paper had a weight of 25 g/m.sup.2, and the CaCO.sub.3 content of the prepared cigarette papers was about 30 wt.-%, based on the total weight of the cigarette paper.

[0233] In particular, the precipitated calcium carbonate prepared in accordance with the inventive process was prepared as described above for Example 2, i.e. a parent solution of manganese(II) sulfate monohydrate was added to a diluted slurry of hydrated calcium oxide having solids content of 13.5 wt.-%, based on the total weight of the slurry, in an amount of 300 ppm (d/d), based on the total dry weight of the hydrated calcium oxide, after slaking but before the carbonating was started.

[0234] Characteristics and physical properties of the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are given in table 4.

TABLE-US-00004 TABLE 4 Example 1 2 (prior art) (inventive) specific surface area BET m.sup.2/g 10.6 10.6 PSD weight median particle size d.sub.50 HELOS Particle size analyzer ?m 2.25 2.17 Sedigraph III Plus ?m 1.39 1.37 R457 (ISO 2469) % 97.1 94.6 Yellowness index (DIN 6167) % 1.3 2.4 Solids content slurry % 35.3 36.9 Viscosity Slurry (Brookfield mPas 380 496 100 rpm) pH slurry 9.1 8.3

[0235] The pulp used for the preparation of the cigarette paper in both cases consisted of 50 wt.-% virgin fibers and 50 wt.-% broke. The virgin fibers consisted of 30% long fibers and 70% short fibers.

[0236] The following machine parameters were used:

TABLE-US-00005 Speed wire: 206 m/min Speed pope: 220 m/min Production: 465 kg/h

[0237] The pulp was combined with water and the following additives: [0238] Cationic starch (2%) at a rate of 2501/h, and [0239] Precipitated calcium carbonate at a rate of 189 kg/h.

[0240] After the combination, the resulting material was refined in a refiner so that the long fibers showed a ? SR of 70.

[0241] After the wet-end, the wet-press and the dryer section the obtained papers showed the properties as described in Table 5:

TABLE-US-00006 TABLE 5 Cigarette paper with Cigarette paper with pigment 1 (prior art) pigment 2 (inventive) Opacity [%] 69.61 70.64 Brightness [%] 92.37 92.43 Yellowness index 3.88 3.80

[0242] It has to be noted that the above values for opacity, brightness and yellowness represent average values that were taken from several trials.

[0243] As can be derived from table 5, the average opacity obtained with the inventive precipitated calcium carbonate was about 71% (70.64%) and 70% (69.61%) of the precipitated calcium carbonate prepared according to the prior art. This means that the use of the inventive precipitated calcium carbonate provides an increase in opacity of about 1.5%.

[0244] The opacity of the obtained cigarette papers comprising the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are also shown in FIG. 3.

[0245] As can be gathered from the data shown in FIG. 3, the inventive method especially leads to precipitated calcium carbonate achieving an increased or optimized opacity when implemented as filler material in cigarette paper compared to a prior art calcium carbonate.

Example 6Paper Study

[0246] Handsheet studies were done simulating copy paper comprising a precipitated calcium carbonate prepared in accordance with the inventive process and a calcium carbonate prepared in accordance with the prior art, respectively. In particular, the precipitated calcium carbonate prepared in accordance with the inventive process was prepared as described above for Example 2, i.e. a parent solution of manganese(II) sulfate monohydrate was added to a diluted slurry of hydrated calcium oxide having solids content of 13.5 wt.-%, based on the total weight of the slurry, in an amount of 500 ppm (d/d), based on the total dry weight of the hydrated calcium oxide, after slaking but before the carbonating was started. Each paper had a weight of 80 g/m.sup.2. The CaCO.sub.3 content of the prepared papers was between 10 and 25 wt.-%, based on the total weight of the paper.

[0247] Characteristics and physical properties of the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are given in table 6.

TABLE-US-00007 TABLE 6 Example 1 2 (prior art) (inventive) specific surface area BET m.sup.2/g 6.1 6.3 PSD weight median particle size d.sub.50 HELOS Particle size analyzer ?m 3.60 3.55 Sedigraph III Plus ?m 2.28 2.29 R457 (ISO 2469) % 96.9 90.3 Yellowness index (DIN 6167) % 1.3 4.8 Solids content slurry % 35.2 38.9 Viscosity Slurry (Brookfield mPas 110 330 100 rpm) pH slurry 8.9 8.6

[0248] Characteristics such as the filler content, opacity as well as whiteness R457 of the obtained papers comprising the precipitated calcium carbonate prepared in accordance with the inventive process and the calcium carbonate prepared in accordance with the prior art are given in table 7.

TABLE-US-00008 TABLE 7 filler content Opacity Whiteness R457 Paper comprising [wt.-%] [%] [%] Prior art PCC 12.42 86.255 86.91 15.93 88.62 87.30 19.89 89.51 87.48 Inventive PCC 9.46 87.74 84.54 14.72 89.25 85.29 19.86 90.89 85.22

[0249] As can be also gathered from the data shown in FIG. 4, the inventive method especially leads to precipitated calcium carbonate achieving an increased or optimized opacity when implemented as filler material in paper compared to a prior art calcium carbonate.